How Many Gallons Is an Above Ground Pool? Chart by Size

An 18 foot round above ground pool with a 48 inch wall holds about 6,372 to 6,691 gallons at a normal fill line. Many charts say 7,646. The difference is depth: most gallon charts never tell you how deep they assumed the water was, and the usual assumption is a pool filled to the top of the wall, which nobody does.

How above ground pools are named

An above ground pool is sold by two numbers. An 18 x 48 is 18 feet across with a 48 inch wall. A 15 x 30 oval is 15 feet wide and 30 feet long, and the wall height is listed separately. The wall height is not the water depth. The skimmer is cut into the top of the wall, and Pool Warehouse notes that the cutout starts only 1 inch below the top of the panel. The water has to sit partway down that opening.

Two sources disagree on how far down, and both have a point.

Pool Warehouse says that on a 48 inch wall the “typical water depth is 42"", and on a 42 inch wall it is 36 inches. The reason is the skimmer: “The water level on your swimming pool should be about halfway up on the skimmer.” Too high and debris floats past it. Too low and the pump can lose prime.

Royal Swimming Pools puts it at 40 inches for a 48 inch wall, 44 for a 52 inch wall and 46 for a 54 inch wall. It subtracts the same 6 inches for the waterline, then adds that “you will build the base of your pool up to have a 2” pool bottom”, so “the water depth ends up being about 8” shallower than the wall height.”

Pool Warehouse subtracts only the gap between the waterline and the top of the wall. Royal also subtracts the sand base. Neither is wrong. So the honest answer is a range, not one number.

Round above ground pool gallons chart

The formula is diameter x diameter x water depth in feet x 5.9. That 5.9 is the same circle factor our pool volume hub uses. The first figure in each cell uses Pool Warehouse’s depth (wall minus 6 inches). The figure in brackets uses Royal’s (wall minus 8 inches).

Your pool sits somewhere between the two numbers in its cell. If your water line sits halfway up the skimmer and your base is thin, use the first. If you have a full 2 inch sand base, use the one in brackets.

A 24 foot round pool with a 48 inch wall holds 11,328 to 11,894 gallons. With a 52 inch wall it holds 12,461 to 13,027. An 18 ft round pool with a 52 inch wall holds 7,009 to 7,328.

Why other charts show bigger numbers

Take the 18 x 48 and run it at three depths.

The brim-full figure overstates the water by 14.3% against a 42 inch fill and 20.0% against a 40 inch fill.

Now look at PoolAndSpa.com’s published example: an 18 x 18 circular pool at 7,646 gallons. That is exactly the brim-full number above. It is a 4.0 foot depth. For an 18 foot pool that describes either a 48 inch wall filled to the very top or a 54 inch wall filled correctly, and the page does not say which. The same article gives a 12 ft round as about 3,400 to 3,800 gallons, which starts about where our 54 inch wall column ends. A chart can look authoritative and still be describing a pool that is not yours.

Oval above ground pool gallons

Ovals use the same 5.9 factor: length x width x depth x 5.9. These are at a 48 inch wall with 42 inches of water.

So a 15 x 30 oval with a 48 inch wall holds about 9,292 gallons at Pool Warehouse’s fill line. If a guide multiplies ovals by 6.7 instead of 5.9, our hub explains why that is an old rounding error.

Even the formula and the chart can disagree

PoolPeople publishes both a formula and a chart. The formula is “diameter x diameter x depth x 5.9”, and every figure in the chart assumes 4 feet 6 inches of water, their deepest pool. The page does not say how full that pool is relative to its wall. Run their own formula at their own depth and the chart comes out a little high every time.

Across all seven sizes they list, the gap runs 2.0 to 2.5%. It is small, and it is not a scandal. It is a reason to compute your own number instead of looking one up. A chart is someone else’s arithmetic with someone else’s assumptions baked in.

Why the depth matters more in a salt pool

Every salt dose scales with gallons. Pounds of salt = (gallons x ppm needed) / 120,000. Bring a fresh 18 x 48 fill to 3,200 ppm and the chart you trust changes the bag count:

• Brim-full chart: (7,646 x 3,200) / 120,000 = about 204 lb
• 42 inch fill: (6,691 x 3,200) / 120,000 = about 178 lb
• 40 inch fill: (6,372 x 3,200) / 120,000 = about 170 lb

A 14 to 20% volume overstatement means 14 to 20% too much salt, here 26 to 34 lb. Adding more is easy. Taking it out is not. Salt is not used up by the cell. Leslie’s notes that it “only needs to be replenished when lost to filter backwashing, splashing, or pool draining.” So the only way to fix an overdose is to drain and refill part of the pool.

Measure your real water depth at the wall, then put your diameter and depth into the pool volume calculator. It answers how many gallons is my pool for your fill line, not a chart’s. Take that number to the pool salt calculator before you open a bag. If you are comparing your above ground pool to an inground one, the average swimming pool size page covers how far apart they run.

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